CA2098901A1

CA2098901A1 - Adhesive composition, and imaging medium comprising this adhesive composition

Info

Publication number: CA2098901A1
Application number: CA002098901A
Authority: CA
Inventors: Iris B. K. Bloom; Richard A. Minns
Original assignee: Polaroid Corp
Current assignee: Polaroid Corp
Priority date: 1992-07-31
Filing date: 1993-06-21
Publication date: 1994-02-01
Also published as: JPH06202315A; KR940002660A; KR0153541B1; JP2643782B2; DE69307618T2; DE69307618D1; EP0581144A2; US5387490A; EP0581144B1; EP0581144A3; US5275914A

Abstract

ADHESIVE COMPOSITION, AND IMAGING MEDIUM
COMPRISING THIS ADHESIVE COMPOSITION
Abstract of the Disclosure A laminar thermal imaging medium is prepared from a first element comprising a first sheet transparent to image-forming radiation and having at least a surface zone or layer of polymeric material heat-activatable upon subjection of the thermal imaging medium to brief and intense radiation, the first element carrying a layer of porous or particulate image-forming substance having cohesivity in excess of its adhesivity for the polymeric heat-activatable layer, and, on the opposed side of the layer of porous or particulate image-forming substance from the surface zone or layer, a first layer of adhesive, and a second element comprising a second sheet carrying a second layer of adhesive. One of the first and second layers of adhesive comprises a polymeric hardenable adhesive comprising a macromolecular organic binder having amino or substituted amino groups, and a photopolymerizable monomer. The first and second elements are laminated together with the first andsecond layers of adhesive in contact with one another, so forming a unitary laminar medium in which the hardenable adhesive remains in its unhardened condition and serves to reduce the tendency for the unitary laminar medium to delaminate on application of stresses to the medium. Only a short lag time, typically about 30seconds, is required between lamination and curing of the hardenable adhesive inorder to provide a strong bond between the two elements following curing.

Description

~" J

C-777~1 ADHESIVE COMPOSiTl(:)N, QND IMAGING MEDIUM
COMPRISIN~; THIS ADIIESIVE COMPOSITION
13ac~k~urid of tlle~iOn This invention relates to an adhesive composition, and to an imaging Inediull~ comprising this adhesive composition More particuiarly, this inventiol1 relales ~o a laminar imaging medium having improved resistance to stress-induced delamillation The provision of images by resort to media wllich rely upon the ~eneration of heat patterns is well known. Thermally iMageable media are particlllarly advantageous inasmuch as they can be imaged without certain of therequirements attending the use of silver halide based media, sucl1 as darkroom processillg and protection against ambient light. Moreover, the use of thermal imagin~ materials avoids the requirements of handling and disposing of silver-containing and other processing streams or effluent materials typically associated with the processing of silver halide based imaging materials.
Various methods and systems for preparing thermally generated symbols, patterns or other images have been reported. Examples of these can be found in U.S. Patent No. 2,616,961 (issued Nov. 4, 1952 to J. Groak); in U.S.
Patent No. 3,257,942 ~issued June 28, 1966 to W. Ritzerfeld, et al.); in U.S. Patent NQ. 3,396,401 (issued Aug. 6, 1968 to K. K. Nonomllra); in U.S. Patent No.
3,592,644 (issued July 13, 1971 to M. N. Vranckel-, et al.); in U.S. Patent No.
3,632,376 (issued ~an. 4, 1972 to D. A. Newman); in U.S. Patent No. 3,924,041 (issued Dec. 2, 1975 to M. Miyayama, et al.); in U.S. Patent No. 4,123,578 (issued Oct. 3 1, 1978 to K. J. Perrington, et al.); in U.S. Patent No. 4,157,412 (issued June 5, 1979 to K. S. Deneau); in United Kingdom Patent Speciflcation No. 1,156,996 publisl1ed July 2, 1969 by Pitney-Bowes, l11C.); and jn lnternational Patent Application No. PCT/US87/03249 of M. R. Etzel (published Ju!-e 16, 1988, as International Publication No. WO 88/04237).

J '~ J ~-ln the produc~ion of a thermally actuatable irnaging material, il may be desirable ancl preferred that an image-formillg substance be confmed between a pair of sheets in ~he form of a laminate. Laminar thermal imaging materials are, for example, described in ~he aforementioned U.S. Patents 3,924,041 and 4,157,412 and in IlIe aforementioned InternatiolIal Patent App!ication No. PCT/US87/03249. It will be appreciated that the sheet elements of a laminar medium will afford protection of the image-forrning substance agaillst the effects of abrasion, rub-off and o~herphysical stil-tluli. In addition, a lamillar medi~ can be handled as a unitary structule, tlIus, obviatir)g the requirement of bringillg the respective sheets of a two-i 0 sheet imaging medium into proper position in a printer or o~her apparatus used for thermal imaging of the medium material.
In the aforementioned International Patent Application No.
PCT/US87/03249, there are described certain preferred embodiments of a high resolution thermal imaging medium, whicll emhodiments include a porous or 15 particulate image-forming substance (e.g., a layer of pigment and binder) conFmed in a laminate structure betweell a pair of sheets. Upon separation of the respective sheets, after laser exposure of portions or regions of the medium, a pair of complemesltary images is obtained. Among the laminar embodiments of International Patent Application No. PCr/US87/03249 are those which inclllde: a 20 first sheet transparent to image-forming radiation and havin~ at least a surface zone or layer of po~ymeric material which iS lleat-aCtiVatable UpOIl subjection of the medium to brief and intense radiation; a layer of porous or particulate image-formin~
substance thereon; and a second sheet laminated and adhesively secured to the ~Irst sheet.
Upon exposure of regions or portions of tl1e medium to brief and intense image-forming radiation, and conversion of absorbed energy to lleat for activatiolI of the lIeat-activatable polymeric material, corresponding regions or portions of the image-formillg substance are caused IO be more firmly attached or lock~d to the f`ilst sheet. Abuttillg re~ions or portions of image-forming substance which are not subjes~tecl lo such image-formhlg radiation are, upon separation of the first and second sheets, removed by the adhesive second sheet, for formation of an image complernelltary to the image on the ~Irst sheet. In preferred thernlal imaging 5 media of the aforementioned International Application, a relense layer is provided o~er the porous or particulate image-forming substance to facilitate proper separation of tile respective first and second sheets and formation of the respective coml)lelllelltary ima~es.
The ,respective images obtained by separating the sheets of an exposed i O thermal imaging medium having an image-forlllillg substance confined ~herebetweell, sucll as a laminar image medium of the type described in the aforemantioned International Application, may exhibit substantially different characteristics. Apart from the irnagewise complementary nature of these images and the relation tl-at each may bear as a "positive" or "negative" of an original, the respective images mayi~ differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of and nature of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be clesirably considered the principal 20 image. The principal image may, however, depending UpOIl ~he aforementioned properties and modes of failure, exhibit dccidedly in~erior properties, sucll as poorer handling characteristics, durability arld abrasion resistance, as compared with the complementary image of secondary importance.
In the production of thermal images from media having "first" and 25 "second" sheets, of the type described hl tlle aforementioned InterllatiorlalApplication, it will oftentimes be preferred, in the case of high density images, thal the prhlcipal image be that which is formed on the second sheet by transfer of noll-exposed regions of coated ima~e-formhlg substance. lt will be recognized that an alterma~ive is to form a high densily image on Ihe first (opposed) sheet by firmly at~aching the image-formillg substance in areas of exposure. This is the case because the medillm provides complementary images and the desired high density image canbe formed on either sheet by addressing the thernially actuatable mediulll according 5 to whicl~ shee~ shall bear the higll density image. Forrnation of a high density image o-- the first sheet is, however, disadvantageous since the areas of high density are created in areas of exposure (by activation of a heat-activatable image-forming zone or layer) and large areas of image-forming substance recluire correspondingly large areas of laser actuation and energy utilization and highly accurate laser scanning and 10 trackhlg. Ç~rrors in tracking will result in discon~inuities (whiteness or voids) by failure to attacll minute regions of image-forming substance and by their removal to the opposed (secontl) sheet upon separation of the sheets. Owing to the psychophysical nature of human vision, minute regions of liglltness (voids) against an e~pansive darkness tend to be noticeable.
It will, thlis, be preferred that a high density image be the result of the transfer in non-exposed regions of coated and continuous regions of image-forming material (with minimal or no discontinuities or coverage voids), rather than the result oi` ~Irnl connection of higll density regions of imaging material by laser-actuated operation of the heat-activatable image-forming surface, where tracking errors 20 increase the possibility of creating noticenble areas of discontitluity (whiteness) against the expallsive high density region.
Inasmuch as the formation of a preferred image in noll-exposed portions of image-forming substance will be the result of the removal of such substance from an opposed sheet with the aid of an adhesive sheet, the adhesive ~5 thereof will serve as a base for tlle image carried by the sheet. The natllre of the adhesive, and especially its pilysical properties, may influellce irnage (luality and certain physical attributes of tlle image, SllCh as the handling properties and durability of the image. If the wrong adhesive is used, the laminar medium matelial may exllibit an ulldesirable tendency ~o delamillate upon subjection ~o cerlain physical stlesses that may be created dIlring a manufacturing operation (e.g., bending, Willdill~, cutting or stamping operations). It may be clesirable in some instances to form a laminar medillm from a pair of endless sheet or web materials ~md to thenS cut, slit or otherwise provide therefrom individual frllm Ullits of predetermined size.
A reciprocal cutting and stamping operation used for the cutting of individual film units may create stress influences in the medium, causing the sheets to separate at the interface of weakest adhesivity -- typically, at the interface where, by thermal achlation, the pre~erential adhesion of the image-forming substance would be I 0 reversed.
In copending Canadian Application No. 2,071,508, there is disclosed an irnproved therMal imaging medium hlcluding a polyllleric hardellable adhesivelayer which in its unhardened condition serves to laminate the sheets of the medium into a unitary mediIJm having a reduced tendency to delamillate upon subjection to 15 physical stresses and whici-, upon subsequent hardening (curing), provides suff~lcient haldness to provide improvements in image halldling and durability; thus, the hardenable adhesive provides a ~Irst degree of adlhesion (hereinafter referred to as "pre-curirlg adhesion") when the two sheets are contacted with one anotl-er or shortly thereafter and a second degree of adhesion (hereinafter referred to as "post-curing 2û adllesion") after tlle adhesive is cured. A preferred type of hnrdellable adhesive for use in shis mediIlm comprises a macromolecIllar organic bhlder; a photopolymerizable ethylenically unsaturated monomer having at least one terminal ethylenic group capable of forming a high molecular weighi polymer by free radical-initiated, chain propagated addition polymerization; and a free radical-generatillg, 25 addition polymeri~ation-initiathlg system activatable by actinic radiation.
This preferred type of hardenable adhesive gives good results.
However, the preferred hardenable adhesive formulations described in the aforementioned copending Canadian Application No. 2,071,5()8, whicll comprise a ~1, J ~

polyfunctiollal acrylate monomer admixecl with a me!hacrylate copolymer, requirea substallticll iag time ~the period between !he lamination of the two sheets and the curillg of the hardenable adhesive~ to ensure ~hat after curing the two sheets adhere sufficielItly to one anotlIer This substantial lag time, wlliclI is of the order of tens 5 of rminlltes, is presumably recluired because it is necessary for the polyfunctional acrylate monomer to diffuse into an adjacent layer of the imaging mediulll in order to provi(le sufficient post-curillg adhesion In some cases, as for example where it is desirecl to carry out curing of the adhesive "in line" with the lamination (i e, when the medilJm is to move continuollsly at a substantial speed along a production line 10 from the lamination to the curisig operations, perhaps via intervening cutting or other stations~, the need for a substantial lag time in order to develop post-curing adhesion is disacivantageous since the production line must be modif~led to provide a long travel for the medium between the lamination station and the curing station, andproviding sucll a long travel will normally involve the provision of numerol~s extra 15 rollers in the prod~lc~ion line, thlls increasing the cost, size and po~ver consllmption of the line It llas now been foulld that, by modifyillg the adllesive compositions describe(l in the aforementioned copending Canadian Application No 2,071,508, the lag time necessary lo develop substantial post-curillg adhesion can be substalltially 20 reducecl Summciry of the Invention Accordingly, this invention provides a method of preparing a lamilIar thermal imaging medium This method comprises the steps of providing a first element comprisillg a flrst sheet transparent to image-2S forming radiation and having at least a surface zone or layer of polymeric materiallleat-activatable upon subjectioll of the tllermal imaging medillm to brief and intellse raciiation, the first element carrying a layer of porolls or particulate image~forming substance having cohesivity in excess of its adhesivity for the polymeric heat-acZivatable layer, and, on the opposed side of the layer ol` porous or particulate image-for~ g snbstance from the surface zone or layer, a first layer of adhesive;
providhl~ a second element comprising a second sheet carrying a second layer of adhesive;
one of the first and second layers of adhesive comprising a polymeric hardel~able adhesive comprising a Macromolecular organic binder having amino or substituted amino groups, and a photopolymerizable monomer;
laminating the first and second elements together with the first and second layers of adhesive in contact with one another and with the firs~ and second shee~s outerrnost and forming a unitary laminar medium in wllich tl-e hardenabl adhesive remains in its unhardened condition and serves to reduce the tendency for the ulli!ary laminar medium to delaminate on application of stresses to Ihe medium;
and subjecting the unitary laminar medium to actinic radiation effective IS to cause polymerization of the photopolymerizable monomer, thus hardening the hardenable ac3hesive into a dllrable polymeric layer This invention also provides a haminar thermal imaging medium, actuatable in response to intense image-forming radiation for productioll of an image, the laminar medium comprising in order a first sheet transparent to the image-forming radiation and having at least a surface zone or layer of polymeric material heat-activatable upon subjection of the therrnal imaging medium to brief and intense radiation;
a layer of porous or particulate image-forming substance having cohesivity in excess of its adhesivi~y for the polymeric heat-activatable layer;a first layer of adhesive affixed, directly or indirectly, to Ille layer of porous or particulate image-formin~ sllbstance;
a second layer of adhesive aclhered to the ~Irst layer of adhesive; and a second sheet covering the layer of porous or particulate image-forn~ substance ancl adhered, via thP first and second layers of adllesive, to the haYCF of image-forming substance, the second sl-eet, upon separation of the fnst and second sheets after exposure to Ihe intense radialion, being adapted to the removal S Iherewilll of unexposed portions of ~he image-forming substance;
one of the first and second layers of adllesive comprisillL~ a polymeric llardenable adhesive comprising a macromolecular organic binder having amino or substitllted amino groups, and a photopolylllerizable monomer, the hardenable adhesive layer being capable in its unhardened condition of reducing the tendency for ihe laminar thermal ima~irlg medium to delaminate on application of physicalstresses to tlle medium and being llardenable to a layer of sufflcient hardness to provide a durable base for the image.
This invention extends to this medillm in botll its uncured and its cured forms.
Finally, this invention provides a photohardenable adhesive composition comprising a mixture of:
a copolymer of at least one dialkylaminoalkyl acrylate or metllacrylate ~vith at least one alkyl acrylate or methacrylate;
a di- or higher functional acrylate or methacrylAte; and a photoinitiator.
Brief Description of the Drawin~s Figure I is a schematic cross-section of a preferred laminar thermally actuatable imaging medium of the present invention; and Figure ? is a schematic cross-section similar to Figure I but showing the medium in a state of partial separation after thermal imaging.
Detailed Description of the Invention As already mentioned, the method of the present invention uses a first element comprising a firs~ sheet transparent to image-forming radiation and having a sul-f`ace zone or layer of polymeric material heaî-/lctivatable upon subjection of the ~herl~ l hna~ medium to brief and intense radiation. The ~Irst element also compl ises a iayer of pOlOUS or particulate ima~e-forming substance having cohesivity hl excess of its adhesivity for the polymeric heat-aclivatilble layer, and, on tlle S opposed sicle of the layer of porous or par~iclllate image-forming subs~ance from the surface zolle or layer, a first layer of adllesive (hereinafter referred to as the "firsl adhesive layer"). The present method also uses a second elemen~ comprising a second sheet carrying a secon(l layer of adhesive (hereillafter referred to as tlle "second adhesive layer"). One of the ~wo adhesive layers comprises a polymeric I O hardenable adhesive comprising a macromolecular organic binder having amino or substitlJted amino (typically diaikylamino) groups, and a photopolymerizable mollomer. The firs~ and second elements are lamina~ed ~ogether wi~h the first and second adhesive Inyers in contact with one another and with the first and secondsheets ou~ermos~, so forming a unitary lamhlar medillm in whicll the hardenable adhesive remains in its wlhardened condition; this unllardened adhesive layer serves to reduce tile tendency for the unitary laminar medilllll ~o delamina~e on application of stresses ~o the medium. Later, the medillm is subjected to actinic radiation effective to cause polymerization of the pho~opolymerizable monomer, thus hardening tlle hardenable adhesive into a durable polymeric layer.
~ely desirably, ~he adllesive layer which does not contain the macromolecular organic binder comprises a polymer having acidic groups, preferably carboxyl grollps. It has been found that contact between the one adhesive layer containing a macromolecular organic binder havhlg amino or substituted amino groups and the other adhesive layer conSaillillg the polymer having acidic groups enables the lag times re(luired before curing of the hardenable adhesive to be greatly reduced, typically to only about 10 to about 30 seconds, while stiil providing good post-curing adhesion between ~he two elemen~s of tile imaging medium. Itl addition, the use of a polymer havil1g acidic grolll)s provides good pre-cl~ring adllesio bclwe~n Ihc lwo elell1el1ts of tl1e imaging me(lillm So far as the developmen~ of post-curing adl)esion is concerlled, eilher of lile l~o aclhesive layers may contaill tlle macrol~loleclllar organic binder having S alnillo or substitllted amino groups. However, for practical reasons it is preferred that tllis macromolecular organic binder be present in the second adhesive layer of lhe second element> and that the first adhesive layer of tl1e first element contairl lhe polymer havirlg acidic groups. As explained in copending Canadian Application No.
2,081,676, when hardenable adhesives containing a macromolecular binder and a 10 polymerizable monomer are incorporated into thermal imaging media, the monomer tends to diffuse into other layers of the media and affect the properties of the rnedia.
Thlls, if sucll a hardenable adhesive is incorporated into the f~lrst element of an imaging medillm of the present invention prior to lamillation of the first and second elements, polyrnerizable monomer may diffuse into the other layers of the first i5 element, with adverse effects on the functioning of tlle f~mal imaging medium. If, on the other hand, the hardenable adhesive is incorporated into the second element of the imaging medium (tllis second element typically comprises only the second aclhesive layer and the second sheet), prior to lalllillatioll of the first and second elemellts the polymerizable monomer cannot diffllse into any other layers where it 20 could affect tlle performance of the final imaging medium.
The present invelltion also ~ives substalltial advantages even whel1 Ihe adhesive layer which does not contain the macromolecular organic binder also does not contain acidic groups. In thermal imagillg media of the type described in copending ~anadian Applications Nos. 2,071,507 and 2,081,676, the cured adhesive25 layer may, depending upon the hardness thereof, fracture or shatter, or separate from the second sheet, on application of stress, as for example where an image formedUpOIl the second sheet is flexed or distorted. Also, in some media of this type, if a sin~le sheet or a pack of sheets of the medium are dropped sharply on one edge, the ~ d 93 '~J ~ ~ ~3 v s~ruc~ure of ~he imagillg medium is disrupted and tlle imaging performance of the mediurll is no longer sa~isfactory. It has been found that the use of first and second adllesive layers, the second of whicll contains amino or substitllte(l amino glOlJpS, ill nccor(lance with tlle presellt invell~ion, substnlltially reduces these problellls.
The aforemelltioned copending C'anildian Application No. 2,0~1,676 describes the use, in a thermal imaging medium, of a barrier or diffusion control layer which resists diffusion of the photopolymerizabZe ethylellicnlly ullsntllrnte(l mollolllcr Iheretllrollgll in order lo prevent this mollolller diffusing into certain other layers of Ihe imaging medilll]l. Such a diffusion control layer canj if desired, be incorporated into lhe imaging medium of the presen~ inventioll between the layer of image-forming substance and the first adhesive layer, but in general it is not necessary to provide a separate diffllsion control layer ~i.e., a diffusion control layer in addition to the first adhesive layer) in the present imaging medium. When themonomer is present in the second adhesive layer, the first adhesive layer will provide i5 some resistance to diffi-sion of the monomer theretl)rougll (thereby acting as a diff;lsion contlol layer), and will thus reduce the tendency for the rnonomer to reacll the layer of image-forming substance, or another lnyer where its presence might advelsely affect the performallce of Ihe imaging meclilllll.
Secondly, the present imaging rnedium will typically be used in a productioll process in which curillg of the hardenable adhesive occurs in lhle witll the lamination step, so that a lag time of only about 10 to about 30 seconds to a few minutes elapses between lamination of tlle first and second elements and curing of tlle hardenable adhesive, and there is little time for the monomer to diffuse into layers where its presence might adversely affect the performance of the imaging medillm. Finally, whell the present imaging mediulll has a second adhesive layercontaining amillo or substitllted amillo groups and a diffl-sible monomer and a first adllesive layer contairling acidic groups, the inleraclions between the amino and acidic groups following lamination of the first and second elements form a strongly I I

`; (3 ~

ionic layer at ~he interface between lhe first ancl second adhesive layers, and it is believed Ihat this strongly ionic layer hinders diffusion of the (typically non-ionic) molll)mer into Ille first adllesive layer and tlle other layers of the first element.
In the preferred form of the present imaging medium in whicll one 5 adllesive layer contains acidic groups, the polymer having acidic groups desirably comprises a copolymer of acrylic or methacrylic acid with at least one acrylate or methacrylate monomer. lhe macromolecular organic bincler llaving amino or substitllted amino groups desirably comprises a polymer having repea~ing UtlitS
derived from a dialkylaminoalkyl acrylate or methacrylate, and is preferably a 10 copolymer of at least one dialkylaminoalkyl acrylate or methacrylate with at least one alkyl acrylate or methacrylate. This copolymer desirably has a glass ~ransition iemperature in the range of from about -10 to about -~50~C, preferably about -10 to about -~30C. An especially preferred group of copolymers of this type comprise from 0 to about 10 parts by weight of butyl acrylate; from about 65 to abollt 95 parts 15 by weight of bu~yl methacrylate; and from about 5 to about 25 parts by weight of N,N-dimethylaminoethyl acrylate. A specific preferred copolymer comprises 5 par~s by weight of butyl acrylate, 82 parts by weight of butyl methacrylate and 13 parts by weight of N,N-dimethylaminoethyl acrylate.
The photopolymerizable monomer used in the present imaging 20 medium desirably comprises a di- or hi~her functional acrylate or methacrylate, preferably a tri- or higller filnctional acrylate or methacrylate, a speciflc preferred monomer being trimethylolpropalle triacrylate.
For optimum performance of the presellt imaging medium, the properties of both the uncllred and cured forms of the hardenable adhesive layer25 shollld be carefully chosen. Prior to curhlg, the hardenable adhesive layer serves to hold the flrst and second elements of the imaging medium ~ogether and to reduce the effects of stress on the medil3m, while after curing the hardenable adhesive layer affects the mechallical properties of the medi~llll, and after imaging acts as a base on ~ ~ i"~

whicll an image rests. It has been ~ound e~cperilnentally that increasing the level of anlillo or subs~ituted amino groups in the hardenable adhesive not only increases the rapid development of post-curillg adhesioll (thus reclucing the lag time reguirecl) but also increases the strength of the cured adhesive, thereby increasillg the resistance 5 of an image formecl thereoll to rellloval or distortion of the areas of the image containing the porous or particulate image-forming substance, if the cured hardenable adhesive is too soi~t, mechanical stress, sucll as that caused by handling of the image, may cause removal of the image-forming substance or distortion of the pels of the image. 1`he resistance of the image to removal or distortion is also affected by the 10 glass transition temperature of the macromolecular orgallic binder used in the hardenable adhesive and the amount of photopolymerizable monomer used therein;
tlle stiffness of the cured hardenable adhesive tends to increase as the glass transition temperature or the concentration of photopolymerizable monomer increases.
I-~owever, care should be taken not to increase the stiffness of tlle cured hardellable 15 adhesive too far, since an excessively stiff cured hardenable adhesive may increase the susceptibility of the imaging medium to the aforementioned sllattering problem.
In Figure 1, there is shown a preferred laminar imaging medium (generally designated 10) of lhe present invention suited to productioll of a pair of lligll resolution ima~es, shown in Figure 2 as imag7es lOa and lOb in a partial state 20 of separation. Thermal imaghlg medium 10 incllldes a flrst elemellt in the form of a first sheet-like or web material 12 (comprishlg sheet material 12a, stress-absorbing layer 12b and lleat-activatable zone or iayer 12c) llavillg superposed tl-ereoll, and in order, porous or particulate image-forming layer 14, release layer 16, first adhesive layer 1~, second, hardenable polymeric adhesive layer 20 and seconcl sheet-like or 25 web material 22.
Upon exposure of the mediIlm 10 to infra-red radiation, 0xposed ps)rtions of image-forming layer 14 are more firmly attached to web material 12, so that, upon separation of the respective sheet-like materials, as sllowll in Figure 2, a pair of images, lOa ancl lOb, is provided. The natllre o~ certain of the layers o~
preferrecl Illermal imaging medium material 10 and their proper!ies are importantly reha~e(l to ~he mallner in whicll the respective images are formed and partiliolled from the medium after exposure. The ~;mctioning of hardenable adhesive layer 20 is 5 important to the reduction of llndesired delamination at the interface between heat-activatable zone or layer 12c ancl porous or partic~late image-forming layer 14 of the preferred thermal imaging medium shown in Figure 1. The various layers of medillm material 10 are described in detail hereinafter.
Web material 12 comprises a transparent material throllgh which 1() imaging medillm 10 can be exposed to radiation. Web material 32 can comprise any of a variety of sheet-like materials, althol-gh polymeric sheet materials will be especially preferred. Among preferred sheet materials are polystyrene, poly(ethylene terephthalate), polyethylene, polypropylene, poly(vinyl chloride), polycarbonate, poly(vinylidene chloride), cellulose acetate, celllllose acetate blltyrate and 15 copolymeric materials such as the copolymers of styrene, butadiene and acrylonitrile, incl-lding poly(styrene-co-acrylonitrile). An especially preferred sheet material from the slandpoints of durability, dimensional stability and handling characteristics is poly(ethylene terephtilalate), commercially available, for example, under the tradename Mylar, of E. 1. du Pont de Nemollrs & Co., Wilmington Delaware, or 20 under the tradename Kodel, of Eastman Kodak ~ompany, Rochester, New 'fork.
The st~ess-absorbing layer 12b is as described in copending Canadian Application No. 2,071,506 and comprises a polymeric layer capable of absorbing physical stresses applied to the imaging medillm 1Ø The stress-absorbing layer 12b provides added protection against delamination of the medium 10 when rigorous 25 physical stresses are applied thereto, and is desirably formed from a compressible or elongatable polyurethane. The stress-absorbing layer 12b is optional and may somelimes be omitted, depending wpon the second adhesive layer 20 ~sed and the stresses to which the medium 10 will be subjected.

-I't-" ~

lleat-activatable ~one or layer 12c provides an essential ~l~nction in ~he imaging of rnedium 10 and comprises a polymeric material whicll is heat activa~able I~pon subjection of the rmedium to brief ancl intense radiation, so that, UpOIl rapid cooling, exposed portions of the surface zone or layer 12c are firmly 5 attaclled to porous or particulate image-~orming layer 14. If desired, when Ihe stress-absorbing layer 12b is omitted, surface ~.one 12c can be a surface portion or region of web material 12, in which case, layers 12a and 12c will be of the same or similar chemical composition In general, it is preferred that layer 12c comprise a discrete polymeric surface layer on sheet material 12a or stress-absorbing layer 12b. Layer 10 12c desirably comprises a polymeric material havillg a softening temperature lower tllall that of sheet rmaterial 12a, so that exposed portions of image-forming layer 14 Ca!~ be firlllly attacl-ed to WGb materhll 12. 1'\ variely of polymeric materi,Ils call be used for this purpose, including polystyrene, poly(styrene-co-acrylonitrile), poly(vinyl butyra~e), poly(methyl methacrylate), polyethylene and poly(vinyl chloride).
The employment of a thin heat-activatable layer 1 2c on a substantially thicker and durable sheet n-aterial 12a permits desired halldling of the web material and desired imaging efficiency. The use of a thin heat-activatable layer 1 2c facilitates the concentration of heat energy at or near the interface bet~/een layers 1 2c and image-formillg layer 14 and permits optimal imagillg effects and reduced energy relluirements. It will be al)preciated that the sensitivity of layer 1 2c to heat activation (or softening) and attachment or adllesion to layer 14 will depend upon the natllre and thermal characteristics of layer 12c and upon the thickness thereof.Stress-absorbing layer 12b can be provided on sheet material 12a by ~he methods described in the aforementioned copending Canadian Application No.
27071,506. Heat-activatable layer 12c can be provided by resort to known coatingmethods. For example, a layer of poly(styrene-co-acrylonitrile~ can be applied to a web of` poly(ethylene terephthalate) by coating from an organic solvent such as methylene chloride. In general, the desired handling properties of web material 12 f ~ 3 will be infl~lellced by tlle nature of slleet material 12a itself, inasltlllcll as layers 12b and 1 2c wil! be coated thereon as thin layers. ~he thickness of web material 12 will clepend upon the desired llanclling characteristics of medium 10 during manufacture and luring imaging and any post-imagillg steps. Thickness will also be dictated in part by the intended use of the image to be carried tllereon and by exposure COnditiOllS, such as the wavelength and power of the exposing source. Typically,web material 12 will vary in thickness from abo~lt 0.5 to 7 mil (about 13 lo 178 ~Im~.
Cood results are obtainecl using, for e~ample, a slleet malerial 1 2a having a thickness of about 1.5 to 1.75 mils (38 to 44 ~m). Stress-absorbing layer 12b will typically have a thickness in the range of about I to 4 ~Im, wllile layer 12c will typically be a layer of poly(styrene-co-acrylonitrile) havillg a ~hickness of about 0.1 to 5 ~lm.
Iieat-activatable layer 12c can incllllle additiYes or agenls providing known beneflcial properties. Adhesiveness-imparting agents, plasticizers, adhesion-redllcing agents, or other agents can be used. Such agents can be usecl, for exampie, to control adhesion between layers 12c and 14, so that undesired separation at the interface tllereof is minimized during manllfactllre of laMinar n-ediulll 10 or during use thereof in a tllermal imaging method or apparatus. Such control also permits the medillm, after imaging and separation of sheet-lil(e web materials 12 and 22, to be partitioned in the manner sllowll in Figure 2.
~mage-forming layer 14 comprises an image-forming substance deposiled onto heat-activatable zone or layer 12c as a porous or particlllate layer or coating. Layer 14, also referred to as a colorant/binder layer, can be formed from a colorant material dispersed in a suitable binder, the colorant being a pigmenl or dye of any desired color, and preferably being substantially inert to Ihe elevated temperatures re~luired for thermal imaging of medillm 10. Carbon black is a particularly advantageous and preferred pigment material. Preferably, the carbonblack material will comprise particles having an average diameter of about 0.01 to 10 ~lm. Althollgll the description herein will refer principally to carbon black, other 3 ~

optically dense s~lbstances, SllCh as graplIite, phthalocyanine pigments ànd other colored pigments can be used. If desired, substances which cilange their opticaldensily UpOlI subjec~ion ~o ~emperatures as hereilI described can also be employed.
The binder for the image-forming substance or layer 14 provides a 5 matfix to form the porous or particulate substance thereof into a cohesive layer and serves to adhere layer 14 to heat-activatable ~one or layer 12c. In general, it will be desired that image-forming layer 14 be adhered to sllrface ~one or layer 1 2c sufficiently to prevent accidental dislocation either during the malIlJfactllre of medium 10 or during the use thereof. Layer 14 shollld, however, be separable (in10 non-exposed regions) from zone or layer 12c, after imaging and separation of webs 12 and 22, so that partitioning of layer 14 can be accomplished in the manner shown in Figure 2.
Image-forming layer 14 can be conveniently deposited onto surface ~one or layer 12c, using any of A number of known coating methods. According to 15 a one embodiment, and for ease in coating layer 14 onto zone or layer 12c, carbon black particles are initially suspended in an inert liquid vehicle, with a binder or dispersant, and the resulting susyension or disperslon is uniformly spread over heat-activatable zone or layer 12c. On drying, layer 14 is adhered as a w~iform image~
~orming layer on the surface thereof. It will be appreciated that tlIe spreading20 characteristics of the suspension can be hlIplovecl by inclllding a surfactAnt, SIICIl as amrnonillm perfluoroalkyl sulfonate, nonionic ethoxylate or the like. Other sLIbstallces~ such as emulsifiers, can be use(l or added to improve tiIe ulIiformity of distriblltion of the carbon black in its suspen(led state and, thereafter, in its spread and dry state. Layer 14 can range in thickness and typically will have a thickness 25 of about 0.1 to about 10 ~Im. In general, it is preferred from the standpoint of image resolution, that a thin layer l~i be employed. Layer 14 should, however, be of sufficient thickness to provide desired and predetermined optical density in theimages prepared from imaging medium 10.

;

.

S~litable binder materials for image-formil1g layer 14 include gelatin, poly(vmyl alcohol), hydroxyethyl cellulose, gum arabic, methyl cellulose, polyvinyipyrrolidone, polyethyloxa~oline, polystyrene !atex and poly(styrene-co-maleic anhydride~. The ratio of pigment (e.g., carbon black) to binder can be in the range of from 40:1 to abollt 1:2 on a weight basis. Preferable, tl1e ratio of pigment to binder will be in the range of from about 4:1 to about 10:1. A preferred binder material for a carbon black pigment material is poly(vinyl alcohol).
If desired, additional additives or agents can be incorporated into image-forming layer 14. Thus, submicroscopic particles, such as cthitin, polytetra-fiuoroetl1ylene particles and/or polyamide can be added to colorant/binder layer 14 to improve abrasion resistance. Such particles can be present, for example, in amounts of from about 1:2 to about 1:20, par~icles to layer solids, by weight.
For tthe prodllctioll of images of high resolution, it will be essential that image-forming layer 14 comprise materials that permit fracture throllgl1 the tllickness of the layer and along a direction substantially orthogonal to the interface between surface zone or iayer 12c and image-forming layer 14, i.e., substantially along the direction of arrows 24, 2arl, 26, and 26', sl1own in Figure 2. It will be appreciated that, in order for images lOa and lOb to be partitioned in the manner shown in Figure 2, image-forming layer 14 will be orthogonally fracturable as described above and will have a degree of cohesivity in excess of its adhesivity for heat-activatable ~one or layer 12c. Thlls, on separation of webs 12 and 22 afterimaging, layer 14 will separate in non-exps)sed areas from heat-activatable layer 12c ancl remain in exposed areas as porous or particulate portions 14a on web 12. Layer 14 is an imagewise disruptibie layer owing to the porous or particulate nature thereof and lhe capacity for the layer to fraclllre or break sharply at parliCle interfaces.
The release layer 16 shown hl Figure I is inclllded in thermal imaging medium iO to facilitate separation of images iOa and lOb according to the mode shown in Figure 2. As described hereinbefore, regions of medillm 10 subjected to ra(liation become more firmly seculed to heat-activatable zone or layer 12c by reason ol' lln:~ heclt aC~iV(l~iOIl of the hlyer by the exposhlg radia(ion. Non-exposed re~ions of` layer 14 remaill only weakly adllered to heat-activatable zone or layer 1 2c and are carried along Wi~ll sheet 22 on separation of sheets 12 and 22. This is accomplished by tlle adllesion of layer 14 to heat-activatable zone or layer 12c, in non-exposed regions, being less than: (a~ the adhesion between layers 14 and 16; (b) the adhesion between layers 16 and 18; (c) the adhesion between layers 18 and 20; (d) the adhesion batween layer 20 and sheet 22; and (e) the cohesivity of layers 14, 16, 18 and 20. The adhesion of sheet 22 to porous or particulate layer 14, throllgil layers î6, 18 and 20, whiie sui`ficient to remove non-exposed regions of porous and particulate layer 14 from heat-activatable zone or layer 12c, is controlled, in exposed areas, by release layer 16 so as to prevent removal of firmly attached exposed portions 14a of layer 14 (attached to heat-activated zone or layer 12c by exposure tl~cl ~
Release layer 16 is designed such that its cohesivity and its adhesion lo either first adllesive layer 18 or porous or particulate layer 14 is less, in exposed regions, than the adhesion of layer 14 to heat-activated zone or layer 1 2c. The result of these relationships is that release layer 16 undergoes an adhesive failure inexposed areas at the interface between layers 16 and 18, or at the interface between ~0 layers 14 and 16; or, as shown in Figure 2, a cohesive failure of layer 16 occurs, SllCh that portions (16b) are present in image lOb and portions (16a) are adhered in exposed regions to porous or particulate portions 1 4a. Portions 1 6a of release layer 16 serve to provide surface protection for the image areas of image lOa against abrasion and wear.
Release layer 16 can comprise a wax, wax-like or resinous material.
Microcrystalline waxes, for example, higll density polyethylene waxes available as aqueolls clispersions, can be used for this pllrpose. Other suitable materials include carnauba, beeswax, paramn wax and wax-like materials such as poly(vinyl stearate), ~9 j J ij .

poly(etllylel)e sebacate), sucrose polyesters, polyalkylelle oxides and dimetl-ylglycol phlhal~l~e. Polyrneric or resinous materials sucll as poly(methyl methacrylate) and copolymers of methyl methacryla~e and monomers copolymerizal)le tllerewith can be employed If desired, hydrophilic colloid materials, sucll as poly~villyl a3cohol~, ~elatin or hydroxyethyl cellulose can be included as polymer binding agents.
Resinous materials, typically coa~ed as latices, can be used arld latices c)f poiy(methyl methacrylate) are especially uset`ul. Cohesivity of layer 16 can be controlled so as to provide tlle desired and predetellllilled fracturing. Waxy or reshlolls layers which are disruptible and which can be fractured sharply at the1~ interfaces of particles thereof can be added to the layer to reduce cohesivity.
E~amples of SllCh particulate materials include, silica, clay particles and particles of polytetra-fluoroethylene.
The f'lrst adhesive layer 18 comprises a polymer having acidic grollps thereoll, preferably carboxyl groups. As discussed above, on contact witll the second adhesive layer 20 (discussed in detail below), flrst adhesive layer 18 serves todevelop rapidly substantial pre-curing arld post-curing adhesion to the second adhesive layer 20, thereby securh~g the ~rst and second elements together to form the unitary laminar imaging medium 10. A specific preferred copolymer for use inlayer 18 is that available as Neocryl 13T 520 from ICI l~eshls (U.S.), Wilmington Massachusetts 01887-0677. This material is an acrylic copolymer conlaining a suf~lcient percentage of free carboxyl groups to perMit solubility in water thatcontains ammonia.
The second adhesive layer 20 of imaging medium 10 comprises a hardenable adhesive layer which is capable of protecting the medium against stresses that would create a delaminatioll of the medium, typically at the interface between zone or layer 12c and image-forming layer 14. The physical stresses which tend to promote delamination and which can be alleviated by hardenable layer 20 can varyand include stresses created by bending the laminar medium and stresses created by J~

whl(lillg, utlwill(lillg, cuttillg, slittillg or stampil-g operations Since hardellable layer 20 carl vary hl compositioll~ it will be appreciatecl illal a particular adhesive may, for example, provide protection ol` ~he medillm against clelamhlation promoted by bendill~ of the medium, while providillg little or no protection agahlst delamination S caused, I`or example, by a slitting or starmping-alld-cu~ting operation, or vice versa As already mentioned, imaging medium 10 is normally prepared by ~lle lamillation ol` flrst and second sheet-like web elements or components, the flrst elemellt or componellt comprising web material 12 carryirlg image-fornlillg layer i 4, release layer 16 and first adhesive layer 18, wllile ~he second element comprises 10 second adhesive layer 20 and second web material 22 The two e!ements can be laminated under pressure, and optionally under heating conditions, to provide the ullitar~ and laminar tllermally actuatable imaging medium 10 of the invention The imaging medium 10 may be subjected to a variety of handlhlg and/or cu~ing procedures before and/or after curil1g of the second adhesive layer 20 15 The lamina~ion of the first and second elements is typically conducted on endless webs of the two elements Following this lamination of endless webs, individual sheets of precletermined size suited, for example, to stacking in a cassette for feeding into a printer apparatus can be prepared from the endless web by die cutting or similar metl1ods Because of the high shear stresses involved in die CUttillg, such die 20 Cllttillg will typically be performed before curillg of the second adhesive layer 20, so tllat the uncured adhesive layer 20 can serve to elimillate or minimize delamina;ion of ihe medium 10 caused by the shear stresses to which the medium 10 is exposed dllrillg die cutting, While applicants do not wish to be bound by any particular theory or 25 mechanism in explanation of the manner in which the second adhesive layer 20 serves to minimize stress-induced delamination of the medium material, it is believed that this layer 20 may serve to absorb physical stresses appiied to medium and thereby re~luce the incidence of delamination Alternativeiy, layer 18 may serve to :

dis~ribute slresses Ihrollgilolll the l~ayer or olherwise prevent applied stresses from behl~ transmitte(l !hrougtl ~he medium and f;om causillg delamination.
Alternatively, ~he second adhesiYe layer 20 may first be c~lred and there~af~er be subjected to cutting operations. Such post-c1Jring cutting is best 5 eft`ec!ed by techni(l1les~ such as slit~ing, which do not place great stress UpOIl the medillm. ~or example, the medium 10 could be laminatecl and cured in-line and thereafter subjected to slitting to trim the edges of the medium before Ihe curecd medi1lm is woulld onto a roll.
As already mentioned, the medium of the present invention has the 10 impOrtAnt advantage that only a short lag time, typically about 10 to about 30 seconds, is required between lamination s~f tlle two elements of the imaging medium and curing in order to develop strong post-curing adhesion. Accordingly, curillg of the mediurrl can conveniently be effected in line without the need to provide a very long travel for the medium between the lamination station and the c1lrillg station.
l 5 On the other hand, the medium of the presen; invention can be formulated to permit a lorlg lag time between lamination of the two elemellts and curing of the hardenable adhesive layer, without adverse effects on the performance of the final, cured medium. Preferred imaging media of the invention llave been found experimentallyto encl1lre lag times of about 18 ho1lrs without adverse effects. Long lag times are 20 advantageous in case it is necessary to halt the line for several hours because of, for example, mechanical or electrical failure, since the long lag time avoids the need to scrap material aiready laminated, but not cured, when the line is halted, and also simplifies bringing the line back into operatioll. Such long lag times are also advantageous if it is desired to effect curing of the medium off line, since the long 25 !aL~ times allow for considerable delays before the curillg must be effected.Cut~ing of the present imaging medi1lm can be effected before or after curing; in some cases, the choice be~ween pre-curillg and post-curing cutting will be determined by the cutting techni~lue employed. If it is desired to cut before cl~rhlL~ SllCIl cul~ing can be perforn ed eitl er in-line wilh lan inalion and curil g or off iine 1 I e lon~ lag times acl ievable witl tl e present imaging medium facilitate off line pre-curing cutting since considerable delays can be tolerated between laminstion an l cutting or between cutting and curing witllout adverse effects on the 5 properties oî ~he f~lnal cure(l medium Preferred n acrormoleclllar binders for use in the second adhesive layer 20 11 ve already been disclJssed above Suitable photopolymerizable ethylenicallyunsaturated monon ers for such compositions include the di- tri and I igl er functiol-al aclylates such as the aforementioned acrylate and methacrylate esters of polyhydric 10 alcohols (e g pentaerythritol triacrylate and trimethylolpropane triacrylate the latter being tl e especially preferre d monomer for use in the present method and imaging medium) Other suitable monomers include etllylelle glycol diacrylate or dimethacrylate or mixtures thereof; glycerol diacrylate or triacrylate; urethaneacryiates; an t epoxy acrylates In general photopolymerizable monomers which 15 provide tack in such compositions or which serve to plasticize the macromolecular bincler will be preferred Those skilled in the art of photopolymerization wiil be aware that most pllotopolymerizable monomers reguire the presence of a photoinitiator for polymerization (curing) of the monomer to occur and tl-us typically the second 20 adhesive layer 20 will comprise sucll a photoinitiator The photoinitiators and concentrations thereof required with various photopolymerizable monomers are well known to those skilled in the art and the conventiollal types and concentrations of pl-otoinitiators can be used in the second adhesive layer 20 Thus the specific preferred photopolymerizable monomer trimethylol-propane triacrylate requires the 25 presence of a free-radical ~eneralill~ photoillitiator for example that sold commercially under the tradename Irgacure 651 by Ciba-Geigy To prevent premature curing of the hardenable adhesive it may be desirable to include a small amoullt of a free radical inhibitor for example a pherlol In general, seconrl ~dllesive layer 20 can be coatecl as a low viscosity sohltivrl ilncl thell dried to a higilly viscous coating. Anti-oxidants can be included, if desire(l. Tilickellers, binders and coating aids can be included to control viscosity and facilitate coath~g to a uniform and adhesive layer. Tack-promoting and plasticizing agenls can be included for their known properties Photohardening of second adhesive layer 20 can be accomplished in known manner by polymeri~ation, using conventional sources of ultraviolet radiation such as carbon arc lamps, commercially available ultra-violet electrodeless bulbs (for example, "D" and "H" bulbs sold by Fusion UV Curing Systems, 7600 Standish Place, Rockville Marylancl 20855-27983, xenon lamps and medillm pressure mercurylamps. The choice of a suitable irradiating source for hardening will also depend on tl-e Ihickl)ess of the layer to be hardene(l.
The thickness of the second adhesive layer can vary and, in general, will be in ~he range of from 0.1 to 50 ~Im. A preferred range of thickness is from 0.5 to 20 ~Im.
As is known in the art, phots~polylnerization systems are oftentimes sensitive to atrmospheric oxygen. The use of photopolymerizable compositions as described above and which are sensitive to o~;ygen can be used to advantage.
Individulllly ClJt units of medium 10 tend, at the edge regions of second adhesive ~0 layer 20 about Ihe perimeter of the laminar mediulll, to be incompletely polymerized and to retain a degree of so~h~ess which reduces the tendency for the medium to delaminate.
The use of hardenable second adhesive layer 20 in medium 10 i5 advantageous from the standpoint of permitting lamination of the components thereof 25 without the requirement of elevated temperatures that may have an adverse influence on other layers or components of the medium. While heat and pressure can be usedto effect the lamination, pressing of the components withollt heat can be used to provide the lamination. Tile use of a hardenable layer 20 that can be cured ullder ambiellt room conditions reduces the reqllired dwell time to achieve lamination and increases manufacturing efficiency.
Upon curing of second adhesive layer 20, mediurn material 10 is ready for imaging. AttaclIment of weakly adherent image-~orming layer 14 to heat-5 activatabie zone or layer 1 2c in areas of exposure is accomplished by (a) absorptionof radiation within the imaging medi~lnl; (b) conversion of the radiation to heat sufficielIt in intensity to heat activate zone or layer 12c; and (c) cooling to more firmly join exposed regions or portions of layer 14 so heat-activatable zone or layer 12c. Thermal imaging medium 10 is capable of absorbing radiation at or near the 10 interface of layer 14 witll heat-activatable zone or layer 12c. This is accomplished by using layers in medium 10 which by their nature absorb radiation and generatethe requisite heal for desired tlIermal imaging, or by including, in at least one of the layers, an agent capable of absorbing radiation of the wavelength of the exposing sollrce. Infrared-absorbing dyes can, for e~ample, be suitably employed for this purpose.
If desired, porous or particulate image-forming substance 14 can comprise a pigment or other colorant material SllCh as carbon black which, as is more completely described hereinafter, is absorptive of exposhIg radiation and which is kno~n in the thermographic imaging field as a radiation-absorbing pigment.
Inasmllcll as a secure bonding or johIillg is desired at the interface of layer 14 and 2û heat-activatable zone or layer 12c, i~ may be preferred in some instances that a radiation-absorbing s~lbstance be incorporated into either or both of image-forming layer 14 and heat-activatable zone or layer l~c.
Suitable radiation-absorbing substalIces in layers 14 and/or 12c, for converting radia~ion into heat, hIclude carbon black, graphite or finely divided~5 pigments such as the sulfides or oxides of silver, bismutll or nickel. Dyes suclI as the azo dyes, xanthene dyes, phthalocyanine dyes or anthraqllinone dyes can also be employed for this purpose. Especially preferred are materials whiclI absorb effciently at the particlllar wavelength of the exposing radiation. In this connection, infra~e(l-absorbillg dyes which absorb in !l~e inflared-ell1ilting regions of lasers which ~Ire (I~,Sir~lbly llSGCI for therlllal ima~ing are espe~ially preferred. Suitable examples o~ in~rared-absorbing dyes for this purpose include the alkylpyrylium-squaryliumdyes, disclose(l in U.S. Patent No. 4,508,81 1, and including 1,3-bisl2,6-di-t-butyl-4ll-S ~hiopyran-4-ylidelle)methyl]-2,4-ciihydroxy-dihydroxide-cyclobLItelle diylium-bis{inner salt~. Other suitable infrared-absorbing dyes include Ihose described in copending Canadian Application No. 2,067,959; in copending Canadian Application No.
2,070,2~0; and hl International Application No. PCT/US92/09992.
Tllerrnal imaging medium 10 can be imaged by creating (in medium 10 10) a therma! pattern according to the informatioll imaged. Exposure sources capable of providing radiation which can be directed onto mediuln 10, and which can be converted by absorption into thermal energy, can be used. Gas discharge lamps, xenon lamps and lasers are exarnples of such sources.
The exposure of mediull- 10 to radiation can be progressive or 15 intermittent. For example, a medium as showrl hl Figute 1 can be fastened onto a rotating drum for exposure of the medium throllgll sheet 12. A radiation spot of higll intensity, sucl- as is emitted by a laser, can be used to expose ~he medium 10 in the direction of rotation of the drum, while the laser is moved slowly in a transverse direction across the web, thereby to (race out A helical path. Laser drivers, designed 20 ~o fire corresponding lasers, can be used to intermittelltly fire one or more lasers in an imagewise and predetermined manner to tl-ereby record informalion according to an original to be imaged. As is shown in Figure 2, a pattern of intense radiation can be directed OlltO medium 10 by exposure to a laser from !he direction of the arrows 24, 24', 26 and 26', the areas between the respective pairs of arrows defining regions 25 of exposure.
If desired, an imaging medium of the invention can be innaged using a moving slit or stencils or masl;s, and by using a tube or other source which emits radhltiOI) COll~imlOUSIy atld WhiCil can be direcled progressively or intcrmittelltly onto medillln 10. Iherlllograpllic copying rmetllods call be used, if desired.
Preferably, a laser or con-bhlatioll of lasers is used to scan llle medium an(l recorcl infurmation in tlle form of very fule dots or pels. Semiconductor diode 5 lasers and YAG lasers having power oulputs surficient to stay within upper andIower exposure ~hreshold values of medium 10 will be preferred. Useful lasers may llave power outputs in tlle range of from about 40 ~o about 1000 milliwatts. An exposure ~hresllold value, as used hereill, refers to a minimal power required to effect an exposure, while a maximum power OUtpllt refers to a power level tolerable by the 10 medilll7l before "burn Ollt" occurs. I,asers are parlicularly preferred as exposing soulces inasrnllcll as medium 10 may be regarded as a threshold-type of rllm; i.e., it possesses higll contrast and, if exposed beyond a certain thresllold vallle, will yield maximllm density, whereas no density will be recorded below the threshold value.Especially pre~erred are lasers whicll are capable of providing a beam sufflciently fine to provide images having resolution as flne as 4,000 - 10,000 dots per inch(160-400 dots per millimeter).
Locally applied heat, developed at or near the interface of image-forming layer 14 and heat-activatab1e zone or layer 12c can be intense (about 400C) ancl serves to effect imaging in Ihe manner described above. Typically, the laser dwell time on each pixel will be less thall one millisecond, and the temperature in exposed regions can be between about 100C and about 1000C.
Apparatus and methodology for forming images from thermally actuatable rnedia such as the rnedium of the present invention are described in detail hl copending Canadian Applications Nos. 2,052,621 and 2,052,929.
Tlle imagewise exposure of mediuln 10 to radiation creates in the medillm latent images which are viewable llpOIl separation of ~he sheets Ihereof (12 arld 22) as shown in Figure 2. Slleet 22 can cornprise any of a variety of plastic materials transmissive of actinic radiation used for the photohardening of r. . i; . ~ t .

pllototlardel~able ildhesive layer 20. A transparelll polyester (e g, poly(ethylene lelephll)alate)) sheel material is preferred. In acldition, sheet 22 will preferably be sllbcoated, or may be corona treated, to promote ~he adhesion thereto of photohardene(l an~l durable layer 20. Preferably, eacll of sheets 12 and 22 will be 5 flexible polymeric sheets.
~ he thermal imaging medium of lhe invention is especially suited to ~he production of hardcopy images produced by medical imaging equipment such as ~-ray equipment, CAT scan equipn-ent, MR ecluipment~ ultrasollnd equipment and so fort'il. As is stated in Neblette's llandbook of Photography and l~eprography, 10 Seventh ~clition, Edited by John ~1. Sturge, Van Nostrand and Reinhold Company, at pp. 55~-559: "The most important sensitome~ric di~ference between X-ray fllmsand films for general photograplly is tlle contrast. X-ray i~llms are designed to produce higll contrast because the density differences of the subject are usually low and increasing these dif-ferences hl the radiograph adds to its diagnostic value ...
15 Radiographs ordinarily contain densities ranging from 0.5 to over 3.0 and are most effectively examilled on an illuminator witll adjustable liL~ht intellsity ... Unless applied to a very limited density range the printing of radiographs on photographic paper is ineffective because of tile narrow range of density scale of papers." The meclillm o~ the present invention can be used to advantage in the production of 20 medical images using printing apparatus, as described in copending Canadiall Application No. 2,052,621, which is capable of providing a large number of gray scale levels.
Tile use of a high number of gray scale levels is most advantageous at high densities inasmuch as human vision is most sensitive to gray scale changes 25 whicll occur at high density. Specifically, the hllman visual system is sensitive to relative change in luminance as a function of dL/L where dL is the change in lumillance and L is the average lumillallce Thus, whell the density is higll, i.e., L
is small, tlle sensi~ivity is high for a ~iven dL whereas if the density is low, i.e., L

is large, then tl)e sensi~ivity is low for a given dL. In accordance with this, the mediilm of tile presellt mvention is especially suited to utilization with equipment capable of providing small stes~s between gray scale levels at the high end of the gray scale, i.e., in the high contrast region of greatest value in diagnostic imaging.
S ~urther, it is desirable that the high density regions of the gray scale spectrum be rendered as accurately as possible, inasmucil as the eye is rnore sensitive to errors which occur in ~ilat region of the spectrum.
rhe medium of tlle present invention is especially suited to the production of higll density images as image lOb, shown in Figure 2. It has been 10 noted previously Zhat separation of sheets 12 and 22 without exposure, i.e., in an unprinted state, provides a totally dense image in colorant Material on sheet 22(image lOb3. The making of a copy entails the use of radiation to cause the image-forming colorant material to be firmly attached to web 12. Then, whell sheets 12and 22 are separated, the exposed regions will aclhere to web 12 while ullexposed 15 regions will be carried to sheet 22 and provide the desired high density image lOb.
Since tlle higll density image provided on sheet 22 is the result of "writillg" on shee 12 with a laser to firmly anchor to sheet 12 ~and prevent removal to sheet 22) those portions of the colorant material whicll are ullwarlted hl image lOb, it will be seen ~llat the amoullt of laser actuation required to produce a high density image can be 20 kept to a minimum. A method for providing a therrrlal image while keeping exposure to a minimum is disclosed and cl~imed hl International Application No.
PCT/US91/06898, Publicalion No. WO 92/09939.
If medium 10 were to be exposed in a manner to provide a high density image on slleet 12, it will be appreciated that the high density gray scale 25 levels would be written on sheet 12 with a shlgle laser at an inefflcient scanning speed or by the hlteraction of a number of lasers, increasing tlle opportunity for tracking error. Because medical images are darker than picture photographs and tracking errors are more readily detected h~ the lligll density portion of gray scale ~ ~ ~ '?~

ievels, a prillting appara~lls, llsing mecli-ll1) 10, wollld need to be complex and expensive to acllieve a comparable level of acc~lracy in the production of a high density meclical irmage on sheet 12 as can be achieved by exposing the medium for prodnction of the high density image on sheet 22 Since image iOb, by reason of its informational content, aesthe~ics or othel-wise, will oftentimes be considered the principal image of the pair of images formed from medium l 0, it may be desired that the thickness of sheet 22 be considerably grea~er, and the sheet 22 thlls more durable, than sheet 12 ~n addilion, it will normally be benef~cial from the standpoints o~ exposllre and energy 10 requirements that sheet 12, throllgh which exposure is effected, be thinner than sheet 22 Asymmetry in sheet thickness may increase the tendency of the medil~m materml to delamillate clllring manllfacturing or halldling operations Utilization of photohardenable adhesive layer 20 will be preferred in medillm 10 particularly to prevent deiamination during manufacture of tl-e medillm If clesired, further protection for the irmage 10b against abrasion and added durability can be achievecl by incllldillg an adclitional layer (not shown) of a thermoplastic malerial between image-forming layer 1~ and sllrface zone or layer12c, which additional layer comprises a polymeric layer fracturable sllbstantially along the exposllre direction and which provicles surface protective portions (over 20 image portions 14b) for improved dllrability of image lOb A laminar thermal ima~ medium inclllding a tl-ermoplastic h~termediate layer to provide surface protection ot` an image prepared therefrolll is disclosed and claimed in copending Canadian Application No 2,071,507 Alternatively, additional d~lrability can be provided to image lOb by 25 depositing a protective polymeric overcoat layer thereon A protected image and methocl therefor are disclosed and claimed in copending Canadian Application No 2 ,070,669 .~

Ihe foliowing Example is no\w given, ths)1lgl1 by way of illustratioll only, to show det~ils of particularly preferred reagents, conditions and techniques use l is~ the method and imaging medium of the ~iresent invention. All par~s, ratios al~d proporlions, except where otherwise h1dicated, are by Y~eight.
EXAMPL.. EI
Onto a first sheet of poly~ethylel1e terephthalatè) of 1.75 mil (44 ~tm) this~kness (ICI Type 3284 film, available froM ICi Americas, Inc., Hopewell, Virginia~ were deposited the following layers in succession:
a 2.4 ~Im tllick stress-absorbing layer of polyurethane (a mixture of 1090% ICI Neotac R-9619 and 10% ICI NeoRez R-9637, both froM ICI Resins (IJ.S.),Wilmington, Massachusetts);
a 1.3 ~m thick heat-activatable layer of poly(styrene-co-acrylonitrile);
a I ~lm thick iayer of carbon black pigment, poly(vinyl alcohol) (PVA), 1,4-butanediol diglycidyl ether, and a fluorochelllical surf~lctal1t (FC-171, I Savailabie from Minnesota Minh~g and Manufacturillg Corporation, St, Paul, Minnesota 55144-1000) at ratios, respectively, of 5:1:0.18/0.005;
a 0.6 ~lm thick release layer comprisin~ polytetrafluoroethylene, silica and hyclroxyethylcellulose (Natrosol +330, available from Aq1lalon Incorporated,Bath, Pennsylvania 18014), at ratios, respectively, of 0.S:1:0.1; and 20a 2.2 ~Im thick layer ot` the aforemenlioned Neocryl BT 520 copolymer containirg acidic groups.
To form the second adhesive layer, 5 parts of butyl acrylate, 82 parts of b1~tyl methacrylate and 13 parts by weight of N,N-dimethylaminoethyl acrylatewere copolymerizecl witlI 2.,2-azobis(2-metllylpropanenitrile) to form a copolyiner 25I avh1g a nulliber average molecular weight of about 40,000 and a glass transitioll temperature of +11C. A coating solution was prepared comprising 11.90 parts of this copolymer, 2.82 parts of trimethylolpropane triacrylate (TMPTA, available as A~eflex TMPTA from CPS Chemical Company, Old Bridge, New Jersey 08857), s ~J ~

0 007 parls of 4-methoxypllenol (a free radical inhibitor), 1 14 parts of 2,2-dill~e~hoxy-2-phellylacetopllellol1e (a photoinitiator, available as Irgacure 651 fron~
Ciba-Geigy Corporation), 0 037 parts of tetrakis~methylene(3,5-di-tert-butyl-~-hydroxyhy(lrocinnamate)} methane (an anti-oxidant, available as Irganox 1010 from Ciba-&eigy Corporation), 0 037 parts of thiodiethylene bis-(3,5-di-tert-butyl-4 hydroxy)llydrocinnamate(ananti-oxidal1t~availableaslrganox 1035fromCiba-~eigy Corpora~ion), and 58 28 parts of ethyl acetate solvent This coating solution wascoated onto 4 mil (101 ~Im) poly(ethylelle terephthalate) film (ICI Type 526 anti-static treated fllrn, available from ICI Americas, Inc, Hopewell, Virginia; this film 10 forms the second web 22 of Ihe imaging mediulll 10) and dried in an oven at about 85C ( I ~5F) to a coating weight of about 9400 mg/m2 to form a hardenable second adhesive layer 20 approximately 10 ~Im thick The first and second poly(ethylene terephthalate) sheets were imll)e(liately brou6ht together with their adhesive layers in face-to-face contact, the 15 ~ mil sheet being in contact with a rotating steel drum ~ rubber roll having a Ourometer hardness of 70-80 was pressed against the 1 75 mil sheet The resultillg web of laminar mediurm was then passed in line, approximately 30 seconds after lamination, under a radio-frequellcy-powered source of ultraviolet radiation, with the 4 mil sheet facing, and at a distance of about 2 5 inches (6 4 cm) from, the source 20 (a Model DRS- 11 I Deco Ray Conveyorized Ultraviolet Curing System, sold by the aforementioned Fusion UV C~lring Systems~, which served to cure she second adllesive layer 20 After curing, the web of imaging medium was passed through a slitting station where edgewise trimming along both edges of the medium was 25 performed in the machine direction The resultant trimmed web was ~llen wound onto a take-up roll Individual sheets of imaging medilull cut from the resultant roll were imaged by laser expos~lre thro~gh the 1 75 mil sheet using high intensity i~ S~ 3 ~

semicorlductor lasers. In each case, the medium was fixed (clamped) to a rotary drl~ Witl-l lhe 4 mil sheet facing the drum. The radiation of semicondllctor lasers was direc~ed ~hrollgh the 1.75 mil sheet in an imagewise manner in response to adigital representation of an original image to be recorded in the medillm. After5 exposure to Ihe high-intellsity radiation (by scanning of the imaging medium or~hogonally to the direction of drum rotation) and removal of the exposed imaging medillm from the drum, the two sheets of the imaging medium were separated to provide a first image on the first, 1.75 mil sheet and a second (and complemerltary) image on the second, 4 mil sheet (the principal image).

Claims

1. A method of preparing a thermal thermal imaging medium which comprises the steps of:
providing a first element comprising a first sheet transparent to image-forming radiation and having at least a surface zone or layer of polmeric material heat-activatable upon subjection of the thermal imaging medium to brief and intense radiation, the first element carrying a layer of porous or particulate image-forming substance having cohesivity in excess of its adhesivity for the polymeric heat-activatable layer, and, on the opposed side of the layer of porous or particulate image-forming substance from the surface zone or layer, a first layer of adhesive;
providing a second element comprising a second sheet carrying a second layer of adhesive;
one of the first and second layers of adhesive comprising a polymeric hardenable adhesive comprising a macromolecular organic binder having amino or substituted amino groups, and a photopolymerizable monomer;
laminating the first and second elements together with the first and second layers of adhesive in contact with one another and with the first and second sheets outermost and forming a unitary laminar medium in which the hardenable adhesive remains in its unhardened condition and serves to reduce the tendency for the unitary laminar medium to delaminate on application of stresses to the medium;
and subjecting the unitary laminar medium to actinic radiation effective to cause polymerization of the photopolymerizable monomer, thus hardening the hardenable adhesive into a durable polymeric layer.

2. A method according to claim I wherein the other one of the first and second layers of adhesive comprises a polymer having acidic groups.

3. A method according to claim 2 wherein the acid groups are carboxyl groups.

4 A method according to claim 2 wherein the second layer of adhesive comprises the polymeric hardenable adhesive comprising a macromolecularorganic binder having amino or substituted amino groups, and the first layer of adhesive comprises the polymer having acidic groups.

5. A method according to claim 1 wherein the macromolecular organic binder having amino or substituted amino groups comprises a polymer having repeating units derived from a dialkylaminoalkyl acrylate or methacrylate.

6. A method according to claim 5 wherein the macromolecular organic binder comprises a copolymer of at least one dialkylaminoalkyl acrylate or methacrylate with at least one alkyl acrylate or methacrylate.

7. A method according to claim 6 wherein the macromolecular organic binder comprises a copolymer of:
from 0 to about 10 parts by weight of butyl acrylate;
from about 65 to about 95 parts by weight of butyl methacrylate; and from about 5 to about 25 parts by weight of N,N-dimethylaminoethyl acrylate.

8. A method according to claim 1 wherein the photopolymerizable monomer comprises a di- or higher functional acrylate or methacrylate.

9. A laminar thermal imaging medium, actuatable in response to intense image-forming radiation for production of an image, the laminar medium comprising in order:
a first sheet transparent to the image-forming radiation and having at least a surface zone or layer of polymeric material heat-activatable upon subjection of the thermal imaging medium to brief and intense radiation;
a layer of porous or particulate image-forming substance having cohesivity in excess of its adhesivity for the polymeric heat-activatable layer;a first layer of adhesive affixed, directly or indirectly, to the layer of porous or particulate image-forming substance;

a second layer of adhesive adhered to the first layer of adhesive; and a second sheet covering the layer of porous or particulate image-forming substance and adhered via the first and second layers of adhesive to thelayer of image-forming substances the second sheet upon separation of the first and second sheets after exposure to the intense radiation being adapted to the removal therewith of unexposed portions of the image-forming substance;
one of the first and second layers of adhesive comprising a polymeric hardenable adhesive comprising a macromolecular organic binder having amino or substituted amino groups and a photopolymerizable monomer the hardenable adhesive layer being capable in its unhardened condition of reducing the tendency for the laminar thermal imaging medium to delaminate on application of physical stresses to the medium and being hardenable to a layer of sufficient hardness toprovide a durable base for the image.

10. An imaging medium according to claim 9 wherein the other one of the first and second layers of adhesive comprises a polymer having acidicgroups.

11. An imaging medium according to claim 10 wherein the acid groups are carboxyl groups.

12. An imaging medium according to claim 10 wherein the second layer of adhesive comprises the polymeric hardenable adhesive comprising a macromolecular organic binder having amino or substituted amino groups and the first layer of adhesive comprises the polymer having acidic groups.

13. An imaging medium according to claim 9 wherein the macromolecular organic binder having amino or substituted amino groups comprisesa polymer having repeating units derived from a dialkylaminoalkyl acrylate or methacrylate.

14. An imaging medium according to claim 13 wherein the macromolecular organic binder comprises a copolymer of at least one dialkylminoalkyl acrylate or methacrylate with at least one alkyl acrylate or methacrylate.

15. An imaging medium according to claim 14 wherein the macromolecular organic binder comprises a copolymer of:
from 0 to about 10 parts by weight of butyl acrylate;
from about 65 to about 95 parts by weight of butyl methacrylate; and from about 5 to about 25 parts by weight of N,N-dimethylaminoethyl acrylate.

16. An imaging medium according to claim 9 wherein the photopolymerizable monomer comprises a di- or higher functional acrylate or methacrylate.

17. An imaging medium according to claim 9 further comprising a release layer disposed between the layer of image-forming substance and the first layer of adhesive, the release layer being adapted to facilitate separation between the first and second sheets and to provide, respectively, first and second images.

18. An imaging medium according to claim 9 wherein the first sheet comprises a polymeric stress-absorbing layer capable of absorbing physicalstresses applied to the imaging medium.

19. A laminar thermal imaging medium, actuatable in response to intense image-forming radiation for production of an image, the laminar medium comprising in order:
a first sheet transparent to the image-forming substance radiation and having atleast a surface zone or layer of polymeric material heat-activatable upon subjection of the thermal imaging medium to brief and intense radiation;
a layer of porous or particulate image-forming substance having cohesivity in excess of its adhesivity for the polymeric heat-activatable layer;a release layer adapted to facilitate separation between the first and second sheets and to provide, respectively, first and second images;

a first layer of adhesive comprising a polymer having acidic groups;
a second layer of adhesive adhered to the first layer of adhesive and comprising a polymer is hardenable adhesive comprising a macromolecular organic binder having amino or substituted amino groups, and a photopolymerizable monomer, the hardenable adhesive layer being capable in its unhardened conditionof reducing the tendency for the laminar thermal imaging medium to delaminate onapplication of physical stresses to the medium and being hardenable to a layer of sufficient hardness to provide a durable base for the image; and a second sheet adapted, upon separation of the first and second sheets after exposure to the intense radiation, to the removal therewith of unexposed portions of the image-forming substance.

20. A photohardenable adhesive composition comprising a mixture of:
a copolymer of at least one dialkylaminoalkyl acrylate or methacrylate with at least one alkyl acrylate or methacrylate;
a di- or higher functional acrylate or methacrylate; and a photoinitiator.

21. A photohardenable adhesive composition according to claim 20 wherein the copolymer has a glass transition temperature in the range of about -10 to about +50°C.